1. Field of the Invention
The present invention relates to a tool holder, such as a chuck, used in connection with a rotary machine such as a drill press, milling machine, lathe, a portable electric or pneumatic machine or a hand drill. More particularly, the invention relates to an improved form of a two-jaw chuck incorporating a differential screw mechanism.
2. Discussion of the Prior Art
The problem of gripping a drill or milling tool in a driven rotary tool holder or chuck typically has been resolved by the use of a so-called three jaw chuck wherein the three jaws are disposed in a circle separated from each other by 120.degree. of arc. In its simplest form, such a chuck may comprise a body having a conical inside surface to support and guide the jaws which may be held apart in a circumferential direction by springs. The chuck body may be threaded onto a drive shaft which also engages the ends of the jaws. Gripping is accomplished by driving the jaws axially as the chuck body is threaded onto the drive shaft. This motion induces a radial component, due to the conical shape of the chuck body, which provides the desired gripping force. Although such chucks are relatively inexpensive they do not have a high degree of precision nor are they capable of exerting high gripping forces on the tool.
Another form of chuck commonly employed is the three jaw geared chuck. This design overcomes the principal problems of the earlier design by providing guideways in the chuck body to control accurately the motion of the jaws and teeth on the jaws which mesh with a gear driven nut mounted on the chuck body. While this design is capable of high precision and adequate gripping force, it comprises a substantial number of precision parts and is therefore relatively expensive to produce.
Despite its cost, the three-jaw geared chuck has proven to be a popular design for drilling machines due to its automatic centering feature, precision and gripping power. However, the art also developed means for self-centering in connection with gripping devices having two jaws. One such means is the differential screw or a screw with left and right hand threads. Such means have been employed in vises and also in tool holders of various designs. Bennett & Dalzell U.S. Pat. No. 31,148 (1861) illustrates the use of a differential screw in a two-jaw tool holder while Butler U.S. Pat. No. 253,581 (1882) discloses a similar mechanism for a hand held vise. Wakeley U.S. Pat. No. 314,087 (1885) discloses a differential screw mechanism for a die or nipple holder while Trout & Ortlip U.S. Pat. No. 318,222 (1885) and Pflugh U.S. Pat. No. 913,912 (1909) use such a mechanism to center a pipe within a pipe threading tool. The use of a differential screw mechanism in various bench vises is disclosed in Werner U.S. Pat. No. 920,797 (1909 ), Harbord U.S. Pat. No. 1,361,577 (1920), and Rowe et al U.S. Pat. No. 4,208,045 (1980).
Barnes U.S. Pat. No. 335,009 (1886) discloses a differential screw mechanism in connection with a centering or eccentric chuck having two jaws. In common with certain of the prior art patents cited above, the Barnes patent discloses one jaw having a Vee-shaped face while the other jaw is flat thereby providing three point contact with the shank of the tool.
Although the prior art discloses a large number of differential screw mechanisms in connection with vises, tool holders and chucks it is evident that none of them is capable of providing the combination of accuracy and gripping power of the three-saw geared chuck without the relatively high cost associated with that design.